Driver device for driving an LED unit

ABSTRACT

A driver device and a corresponding method for driving a load, in particular an LED unit comprising one or more LEDs. The proposed driver device comprises power input terminals for receiving a rectified supply voltage from an external power supply, power output terminals for providing a drive voltage and/or current for driving a load, a half bridge unit comprising a first and second switching element coupled in series between a high voltage node and a low voltage node and having a switch node between the first and second switching elements, a buck-boost input filter unit comprising a first inductor and a series diode coupled between a power input terminal and the half bridge unit, a buck output filter unit comprising a second inductor coupled between the half bridge unit and a power output terminal, an energy storage unit, and a control unit for controlling the switching elements.

FIELD OF THE INVENTION

The present invention relates to driver device for driving a load, inparticular an LED unit comprising one or more LEDs. Further, the presentinvention relates to a light apparatus.

BACKGROUND OF THE INVENTION

In the field of LED drivers for offline applications such as retrofitlamps, solutions are demanded to cope with high efficiency, high powerdensity, long lifetime, high power factor and low cost, among otherrelevant features. While practically all existing solutions compromiseone or the other requirement, it is essential that the proposed drivercircuits properly condition the form of the mains power into the formrequired by the LEDs while keeping compliance with present and futurepower mains regulations. Of critical importance is to guarantee amaximum light perceptible flicker (preferably zero) at the same timethat the power factor is maintained above a certain limit.

Further, in off-line converters, energy from the power mains often needsto be synchronously drawn in proportion to the supplied voltage waveformin order to achieve high power factor and low harmonic distortion. Powerconverter architectures with an independent preconditioner stage aretraditionally employed to best accomplish this task without compromisingthe proper form of the energy to be delivered to the load.

Typically, two series connected power stages are employed to obtain highpower factor while keeping the output power constant throughout a mainscycle (or supply cycle, i.e. the cycle of the mains voltage or thesupply voltage). In those architectures the first stage shapes themains' current and the second stage performs the power conversion to theload.

Nonetheless, for reasons related to complexity and cost, simplifiedpowertrain solutions are adopted known conventionally as single-stage,where either of the two stages may essentially not be incorporated. As aconsequence of such simplification, the aforementioned requirements maybe largely compromised and/or the converter performance highly degraded,particularly in terms of size, reliability and lifetime. The latter isusually mainly attributed to the need of using a bulky low frequencystorage capacitor in parallel to the load when constant output powerdelivery is to be guaranteed.

Single stage solutions are common in literature. One reference exampleis given in the work of Robert Erickson and Michael Madigan, entitled“Design of a simple high-power-factor rectifier based on the flybackconverter”, IEEE Proceedings of the Applied Power ElectronicsConferences and Expositions, 1990, pp. 792-801.

An intermediate solution laying half-way between the two-stage andsingle-stage approaches is the single-stage converter with integratedpreconditioner. Such solutions can feature reduced component count andhigh power density while keeping compliance with both load and powermains requirements. Other embodiments with a single power convertingstage allow high power factors (HPF) by means of integrating a boostconverter operating in discontinuous conduction mode. These convertersactually combine the above mentioned two power conversion stages.

A HPF converter for compact fluorescent lamps is described in“High-Power-Factor Electronic Ballast with Constant DC-Link Voltage”, byRicardo de Oliveira Brioschi and José Luiz F. Vieira, IEEE Transactionson Power Electronics, vol. 13, no. 6, 1998. Here, a half bridge isshared by a boost converter and an LC parallel resonant converter, whichis operated above resonance in order to obtain zero voltage switching(ZVS). To further support ZVS the bus voltage is controlled constant.Such a HPF converter, however, typically requires a large bus capacitorand an output rectifier and has only narrow supply voltage and load(drive) voltage ranges.

Another example of integrated power stages is the work of R.Venkatraman, A. K. S. Bhat and Mark Edmunds, entitled “Soft-switchingsingle-stage AC-to-DC converter with low harmonic distortion”, IEEETransaction on Aerospace and Electronic Systems, vol.36, no.4, October2000. In this work, a high frequency transformer isolated, Zero VoltageSwitching (ZVS), single stage AC-DC converter with high power factor andlow harmonic distortion is presented and analyzed for a constant powerload.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a driver device fordriving a load, in particular an LED unit comprising one or more LEDs,particularly providing a high power factor, a virtually constant load,small size, high efficiency, long lifetime and low cost. Further, it isan object of the present invention to provide a corresponding lightapparatus.

According to an aspect of the present invention a driver device isprovided comprising:

-   -   power input terminals for receiving a rectified supply voltage        from an external power supply,    -   power output terminals for providing a drive voltage and/or        current for driving a load,    -   a half bridge unit comprising a first and second switching        element coupled in series between a high voltage node and a low        voltage node and having a switch node (59) between said first        and second switching elements,    -   a buck-boost input filter unit comprising a first inductor and a        series diode coupled between a power input terminal and said        half bridge unit,    -   a buck output filter unit comprising a second inductor coupled        between said half bridge unit and a power output terminal,    -   an energy storage unit, and    -   a control unit for controlling said switching elements.

According to another aspect of the present invention a light apparatusis provided comprising a light assembly comprising one or more lightunits, in particular an LED unit comprising one or more LEDs, and adriver device for driving said light assembly as provided according tothe present invention.

Preferred embodiments of the invention are defined in the dependentclaims. It shall be understood that the claimed light apparatus hassimilar and/or identical preferred embodiments as the claimed device andas defined in the dependent claims.

The present invention is based on the idea to integrate and control abuck-boost converter into a synchronous buck converter. To the poweroutput terminals a load (e.g. a HV LED unit) is connected. Further, tothe power input terminals a mains filter capacitor is preferablyconnected. In this way, the present invention provides both a constantdrive current and power factors of 0.9 or higher.

Compared to two-stage offline drivers for LEDs the problems of highcost, complexity and large component count, which are needed to keepcompliance with the requirements of both power mains and load, areaddressed and solved since the proposed driver device and method featuresimplicity and reduced component count, wherein preferably conventionalcomponents are used.

Compared to single-stage offline drivers for LEDs the following problemsare addressed. A large low frequency capacitor can be omitted byallowing a smaller low frequency storage capacitor voltage vary duringthe supply cycle or mains cycle (e.g. 20 . . . 80%) still while keepingthe output current constant. This in turn translates into smaller size,longer lifetime and more reliable, particularly at high temperatureoperation. Further, reduced component count is needed compromising therequirements of neither power mains nor load. This is achievedintrinsically by the operation of the power stage with integratedpreconditioning function. Still further, an increased converterefficiency, typically limited in most single-stage solutions,particularly for retrofit bulb lighting applications, can be achieved,particularly by virtue of ZVS (Zero Voltage Switching) operation asproposed in a preferred embodiment. Finally, even with the use of alarge low frequency storage capacitor, single stages may not fullyeliminate perceptive flicker. The proposed solution enables constantoutput current and hence perceptive flicker can be completely avoided.

According to the present invention there are various basicconfigurations of the buck-boost integrated synchronous buck converterprovided as different preferred embodiments that cope with various loadand input voltage ranges. All of them can be controlled over full loadrange down to virtually zero load current by means of manipulating theduty cycle only or the switching frequency or by burst mode operation.

The supply voltage may be a rectified periodic supply voltage providedby a power supply. In case an AC mains voltage is provided as inputvoltage to the power supply (or the power input terminals), e.g. from amains voltage supply, a rectifier unit is preferably used (as part ofthe driver device or as an external unit coupled to the power inputterminals) for rectifying a provided AC input voltage, e.g. a mainsvoltage, into the (rectified periodic) supply voltage. Such a rectifierunit may, for instance, comprise a generally known half-bridge orfull-bridge rectifier. The supply voltage thus has the same polarity foreither polarity of the AC input voltage.

Alternatively, if e.g. such a rectified periodic supply voltage isalready provided at the power input terminals, e.g. from a rectifier(representing said external power supply) provided elsewhere, no furtheror only general elements (like e.g. an amplifier) are coupled to thepower input terminals for shaping the provided supply voltage.

There are various embodiment of the proposed driver device whichdistinguish mainly by the coupling of the various elements of the driverdevice.

In one embodiment a high power input terminal is coupled to the highpower terminal of the half bridge unit and a low power input terminal iscoupled to the buck-boost input filter unit. Preferably, in thisembodiment the input terminal of said buck-boost input filter unit iscoupled to the low power input terminal and wherein the output terminalsof said buck-boost input filter unit are coupled to the intermediatenode of the half bridge unit and either the energy storage unit or thehigh power output terminal.

In another embodiment a low power input terminal is coupled to the lowvoltage node of the half bridge unit and a high power input terminal iscoupled to the buck-boost input filter unit. Preferably, in thisembodiment the input terminal of said buck-boost input filter unit iscoupled to the energy storage unit or the low power output terminal andwherein the output terminals of said buck-boost input filter unit arecoupled to the intermediate node of the half bridge unit and the highpower input terminal.

For the coupling of the energy storage unit various embodiments exist.For instance, in one embodiment said energy storage unit is coupledbetween a low power input terminal and said low voltage node of saidhalf bridge unit, while in a different embodiment said energy storageunit is coupled between a high power input terminal and said highvoltage node of said half bridge unit. In still another embodiment saidenergy storage unit is coupled between output terminals of said buckoutput filter unit.

The various embodiments are provided for use in different applicationsand different voltages, and are directed to achieve certain aims. Often,a trade-off is to be made to select the optimum embodiment.

Said energy storage unit comprises a charge capacitor, in particular asingle capacitor.

Advantageously, an input decoupling capacitor coupled between the powerinput terminals and/or an output decoupling capacitor coupled betweenthe power output terminals are additionally provided for high frequencydecoupling. Particularly, the input decoupling capacitor is advantageousif the rectified supply current must go negative as is the case in someembodiments.

The switching elements together form a half bridge (also calledswitching unit) in one embodiment. But generally, the switching elementscan be implemented in various ways, e.g. including transistors (e.g.MOSFETs) or other controlled switching means.

Preferably, the control unit is adapted for keeping the output currentconstant, to keep the voltage across the energy storage element below apredetermined threshold and/or to shape the input current. Zero voltageswitching of the switching elements is arranged for by the design(components) of the proposed driver device. The tasks of the control arekeeping the output current constant; possibly according to a referencecurrent (set point), to keep the bus voltage (i.e. the voltage acrossthe energy storage element) below a preset limit and/or to shape theinput current.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

In the following drawings:

FIG. 1 shows a schematic block diagram of a known two stage driverdevice,

FIG. 2a shows a schematic block diagram of a known single stage driverdevice with input storage capacitor,

FIG. 2b shows a schematic block diagram of a known single stage driverdevice with output storage capacitor,

FIG. 3 shows schematic block diagrams of two embodiments of a firstconfiguration of a driver device according to the present invention,

FIG. 4 shows schematic block diagrams of two embodiment of a secondconfiguration of a driver device according to the present invention,

FIG. 5 shows schematic block diagrams of two embodiment of a thirdconfiguration of a driver device according to the present invention,

FIG. 6 shows diagrams of voltages and currents during one low frequencycycle in an embodiment of the first configuration of the proposed driverdevice,

FIG. 7 shows diagrams of various currents during one high frequencycycle in an embodiment of the first configuration of the proposed driverdevice,

FIG. 8 shows diagrams of voltages and currents during one low frequencycycle in an embodiment of the second configuration of the proposeddriver device,

FIG. 9 shows diagrams of various currents during one high frequencycycle in an embodiment of the second configuration of the proposeddriver device,

FIG. 10 shows diagrams of voltages and currents during one low frequencycycle in an embodiment of the third configuration of the proposed driverdevice,

FIG. 11 shows diagrams of various currents during one high frequencycycle in an embodiment of the third configuration of the proposed driverdevice,

FIG. 12 shows a first embodiment of a control unit of the proposeddriver device,

FIG. 13 shows the switching signal of a first switching element of thehalf bridge unit, and

FIG. 14 shows a second embodiment of a control unit of the proposeddriver device.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a known two stage driver device 10 is schematicallyshown in FIG. 1. Said driver device 10 comprises a rectifier unit 12, afirst stage preconditioning unit 14 coupled to the output of therectifier unit 12, a second stage conversion unit 16 coupled to theoutput of the first stage preconditioning unit 14 and a charge capacitor18 coupled to the node 15 between said first stage preconditioning unit14 and said second stage conversion unit 16. The rectifier unit 12preferably comprises a rectifier unit, such as a known full-wave bridgeor half-wave bridge rectifier, for rectifying an AC input voltage V20provided, e.g., from an external mains voltage supply 20 into arectified voltage V12. Load 22 is, in this embodiment, an LED unitcomprising two LEDs 23 is coupled to the output of the second stageconversion unit 16 whose output signal, in particular its drive voltageV16 and its drive current 116, is used to drive the load 22.

The first stage preconditioning unit 14 preconditions the rectifiedvoltage V12 into an intermediate DC voltage V14, and the second stageconversion unit 16 converts said intermediate DC voltage V14 into thedesired DC drive voltage V16. The charge capacitor 18 is provided tostore a charge, i.e. is charged from the intermediate DC voltage V14,thereby filtering the low frequency signal of the rectified voltage V12to ensure a substantially constant output power of the second stageconversion unit 16, in particular a constant drive current 116 throughthe load 22. These elements 14, 16, 18 are generally known and widelyused in such driver devices 10 and thus shall not be described in moredetail here.

Generally, the driver device 10 complies with the aforementioned demandof high power factor and low flicker at the expense of larger spacerequirements and cost, which might be drastically limited particularlyin retrofit applications. The size of the first stage preconditioningunit 14 may be mainly determined by the associated passive components,particularly if it comprises a switched mode power supply (SMPS), e.g. aboost converter, operating at low or moderated switching frequency. Anyattempt to increase the switching frequency so as to reduce the size ofthese filter components may yield a rapid increase of energy losses inthe hard-switched SMPS and hence the need of use of larger heat sinks

Embodiments of known single stage driver devices 30 a, 30 b areschematically shown in FIG. 2a and FIG. 2b . Said driver device 30comprises a rectifier unit 32 (that may be identical to the rectifierunit 12 of the two stage driver device 10 shown in FIG. 1) and aconversion unit 34 (e.g. flyback converter for the embodiment shown inFIG. 2b or a buck converter for the embodiment shown in FIG. 2a )coupled to the output of the rectifier unit 32. Further, in theembodiment shown in FIG. 2a a charge capacitor 36 a (representing a lowfrequency input storage capacitor) is coupled to the node 33 betweensaid rectifier unit 32 and said conversion unit 34 is provided. In theembodiment shown in FIG. 2b the charge capacitor 36 b (representing alow frequency output storage capacitor) is coupled to the node 35between said conversion unit 34 and the load 22. The rectifier unitrectifies an AC input voltage V20 provided, e.g., from an external mainsvoltage supply (also called power supply) 20 into a rectified voltageV32. The rectified voltage V32 is converted into the desired DC drivevoltage V34 for driving the load 22.

The storage capacitors 18 (in FIG. 1) and 36 a, 36 b (in FIGS. 2a, 2b )are mainly provided to filter out the low frequency component of therectified voltage V12 in order to allow for a constant current into theload. Such capacitors are therefore large, particularly when placed inparallel to the load and when such load is an LED.

Driver devices as shown in FIGS. 1 and 2 are, for instance, described inRobert Erickson and Michael Madigan, “Design of a simplehigh-power-factor rectifier based on the flyback converter”, IEEEProceedings of the Applied Power Electronics Conferences andExpositions, 1990, pp. 792-801.

Most of those single stage driver devices 30 a, b can, althoughfeaturing a lower number of hardware components compared to two stagedriver devices as exemplarily shown in FIG. 1, generally not offer ahigh power factor and a low perceptible flicker simultaneously due tolimitations in the size of the charge capacitor, which must filter outthe low frequency component of the AC input voltage. In addition, singlestage driver devices may critically compromise size, lifetime and themaximum temperature operation of the load (e.g. a lamp) due to the useof large storage capacitors used to mitigate perceptible flicker.

FIGS. 3 to 5 depict several embodiments of three different basicconfigurations of a driver device according to the present invention.Each configuration (embodiments of configuration 1 being shown in FIG.3, embodiments of configuration 2 being shown in FIG. 4, embodiments ofconfiguration 3 being shown in FIG. 5) is represented by two differentembodiments of the proposed driver device including a buck-boostintegrated buck converter. The embodiments of each configuration areused for connecting the rectified supply voltage to either a first (top)switching element or to a second (bottom) switching element. Theconfigurations differ in operation in terms of the in- and outputvoltage range they support, as well in terms of their component stressregarding peak voltage and RMS currents.

All three configurations show a self-stabilizing behaviour, which allowscontrolling the output current constant at predetermined voltage rippleacross the (low frequency) energy storage unit (e.g. storage capacitor).The boost inductor (i.e. the first inductor Lm of the buck-boost inputfilter unit) is designed for discontinuous conduction mode, for whichpurpose an additional diode (Dm) is preferably coupled in series to saidboost inductor. Further, ZVS is possible for both switching elements andboth transitions of the switching element series coupling (i.e.switching elements 60, 61 forming a halfbridge unit), which isdetermined by the buck inductor (i.e. the second inductor Lo or Lc ofthe buck output filter unit). Still further, all configurations aresuitable for high voltage loads, e.g. high voltage strings of LEDs. Theembodiment of type 1 (i.e. the embodiments shown in FIGS. 3a, 4a, 5a )and of type 2 (i.e. the embodiments shown in FIGS. 3b, 4b, 5b ) withineach configuration are thus to a large extent equivalent, for whichreason mainly the embodiments of type 1 will mainly be described in thefollowing.

The diagrams shown in FIGS. 6 to 11 refer to the three configurationsshown in FIGS. 3, 4 and 5. They illustrate both low frequencysteady-state waveforms (FIGS. 6, 8, 10) and high frequency switchingwaveforms (FIGS. 7, 9, 11) of the first, third and fifth embodimentsshown in FIGS. 3a, 4a, 5a . In all cases, an LED load including a seriesconnection of LED units is chosen to operate at 10 W constant power,meaning that the output current must be constant. The rectified AC inputsignal refers to the European mains. Other type of loads and powersupplies are possible as well. In all illustrated embodiments herein,the resulting power factor (or PF) is higher than or equal to 90%,whereas the total harmonic distortion (or THD) is lower than 50%.

It should be noted that the duty cycle refers to the first (top)switching element 60, e.g. 100% duty cycle implies that the first (top)switching element 60 is always on (closed) whereas the second (bottom)switch 61 is always off (open).

A first embodiment of a driver device 50 a according to the presentinvention is schematically shown in FIG. 3a . It comprises power inputterminals 51, 52 for receiving a rectified supply voltage vr from anexternal power supply 20 (e.g. a mains voltage supply providing a mainsvoltage vm) which is preferably rectified by a rectifier 62. The driverdevice 50 a further comprises power output terminals 53, 54 forproviding a drive voltage vo and/or current io for driving a load 22.

A half bridge unit 70 (also called switching unit or half bridge)comprising a first 60 and second 61 switching element is coupled inseries between a high voltage node 57 and a low voltage node 58 andforms a switch node 59 between said first and second switching elements60, 61. A buck-boost input filter unit 71, comprising a first inductorLm and a series diode Dm coupled in series to the first inductor Lm, iscoupled between a power input terminal, here the low power inputterminal 52 and said half bridge unit 70. A buck output filter unit 72comprising a second inductor Lo is coupled between said half bridge unit70 and a power output terminal, here the low power output terminal 54.

In this embodiment the output terminals 55 b, 55 c of the buck-boostinput filter unit 71 are coupled to the intermediate node 59 and anenergy storage unit 73 (which is preferably a single storage capacitorCs), which in turn is coupled the low voltage node 58 of the half bridgeunit 70. The input terminals 56 a, 56 b of the buck output filter unit72 are coupled to the intermediate node 59 and the low voltage node 58of the half bridge unit 70. The output terminals 56 c, 56 d of the buckoutput filter unit 72 are coupled to the power output terminals 53, 54.The high power input terminal 51 is coupled to the high voltage node 57of the half bridge unit 70. The low power input terminal 52 as well asthe input terminal 55 a and the output terminal 55 c of the buck-boostinput filter unit 71 are coupled to a reference potential, preferablyground potential.

A control unit 64 (e.g. a controller, processor or computer that isappropriately designed or programmed) is provided for controlling saidswitching elements 60, 61.

For high frequency decoupling an (optional) input decoupling capacitorCm coupled between the power input terminals 51, 52 and an (optional)output decoupling capacitor Co coupled between the power outputterminals 53, 54 are additionally provided in this embodiment.Preferably the input decoupling capacitor Cm is used if the rectifiedsupply current must go negative as is the case in some embodiments (ase.g. shown in FIG. 7).

A second embodiment of a driver device 50 b according to the presentinvention is schematically shown in FIG. 3b . Compared to the firstembodiment of the driver device 50 a the energy storage unit 70 iscoupled between the high power input terminal 51 and the high voltagenode 57 of said half bridge unit 70. Further, in this embodiment the lowpower input terminal 52 is coupled to the low voltage node 58 of thehalf bridge unit 70 and a high power input terminal 51 is coupled to thebuck-boost input filter unit 71. Still further, the input terminal 55 aof said buck-boost input filter unit 71 is coupled to the energy storageunit 73 and the output terminals 55 b, 55 c of said buck-boost inputfilter unit 71 are coupled to the intermediate node 59 of the halfbridge unit 70 and the high power input terminal 51. The high powerinput terminal 51 as well as the input terminal 55 a and the outputterminal 55 c of the buck-boost input filter unit 71 are coupled to areference potential, preferably ground potential, in this embodiment.

According to the first and second embodiments of the driver device 50 a,50 b the low frequency storage capacitor Cs is connected at the outputof the buck-boost converter, formed essentially by the first inductorLm, diode Dm, and the two switching elements 60, 61, i.e. between thesource electrode of the bottom switch 61 and ground. The load 22 isconnected at the output of the buck converter, formed essentially by thesecond inductor Lo and the two switching elements 60, 61, i.e. inparallel to the capacitor Co and in series to the inductor Lo, whichoperates in bidirectional mode, thereby enabling full ZVS operation inboth switches.

FIGS. 6 and 7 show signal diagrams of the first embodiment of a driverdevice 50 a as shown in FIG. 3a during one mains cycle. For the exampleof the steady-state waveforms shown in FIG. 6 and the high-frequencywaveforms at phase angle π/2 from mains cycle shown in FIG. 7 thefollowing values apply: vm=220 Veff, 50 Hz, 1 MHz switching frequency,Lm=Lo=300 μH, Po=10 W, vo=150V, Cs=1 μF, PF=90%, THD=47%, maximumvoltage stress across switches=632V. The term “av” refers to the averagecomponent over a switching cycle. d indicates the duty cycle.

The ZVS operation is shown in the high frequency switching waveforms ofdepicted in FIG. 7 for the case of π/2 phase angle corresponding to themains cycle. Capacitors Cm and Co filter these high switching frequencysignals to prevent them at the input and output, respectively. Themaximum voltage stress across the switches is the sum of both therectified supply voltage vr and the storage capacitor voltage vc.

As depicted in FIG. 6, this configuration is characterized by the use oflow frequency storage capacitors rated to a voltage that is at leasthigher than the rectified supply voltage peak. Such capacitors can be aslow as 1 μF (i.e. 100 nF/W) and still guarantee a constant output powerand a high power factor. By virtue of the step-down conversion, the loadvoltage can be substantially lower than the supply peak voltage (e.g.150V in case of European mains supply). The rectified input current itnever falls to zero due to the continuous current flow from the inputvoltage supply vm directly to the load 22.

A third embodiment of a driver device 50 c according to the presentinvention is schematically shown in FIG. 4a . This embodiment issubstantially identical to the embodiment of the driver device 50 a, buthere in this embodiment the position of load 22 (together with theoutput decoupling capacitor Co) is exchanged with the position of theenergy storage unit 73. In particular, the energy storage unit 73 iscoupled to the output terminals 56 c, 56 d of the buck output filterunit 72. The high power output terminal 53 is coupled to the outputterminal 55 c of the buck-boost input filter unit 71 and the low poweroutput terminal 54 is coupled to the low voltage node 58 of the halfbridge unit 70 (in which the second inductor is indicated by Lc insteadof Lo here to indicate that the second inductor Lc is coupled in serieswith Cs here, while Lo is coupled to Co in the first and secondembodiments.

A fourth embodiment of a driver device 50 d according to the presentinvention is schematically shown in FIG. 4b . This embodiment issubstantially identical to the embodiment of the driver device 50 b, butalso in this embodiment the position of load 22 (together with theoutput decoupling capacitor Co) is exchanged with the position of theenergy storage unit 73. The energy storage unit is coupled to the outputterminals 56 c, 56 d of the buck output filter unit 72. The high poweroutput terminal 53 is coupled to the output terminal 55 c of thebuck-boost input filter unit 71 and the low power output terminal 54 iscoupled to the low voltage node 58 of the half bridge unit 70.

Thus, according to the third and fourth embodiments the load 22 isconnected at the output of the buck-boost converter (i.e. in parallel toCo and between the source electrode of the bottom switching element 61and ground). The low frequency storage capacitor Cs is connected at theoutput of the buck converter (i.e. in series to inductor Lc), whichoperates in bidirectional mode, thereby enabling full ZVS operation inboth switches.

FIGS. 8 and 9 show signal diagrams of the third embodiment for anembodiment of a driver device 50 c as shown in FIG. 4a during one mainscycle. For the example of the steady-state waveforms shown in FIG. 8 andthe high-frequency waveforms at phase angle π/2 from mains cycle shownin FIG. 9 the following values apply: vm=220 Veff, 50 Hz, 1 MHzswitching frequency, Lm=200 μH, Lc=400 μH, Po=10 W, vo=400V, Cs=2 μF,PF=91%, THD=44%, maximum voltage stress across switches=711V. The term“av” refers to the average component over a switching cycle. d indicatesthe duty cycle.

The ZVS operation is shown in the high frequency switching waveforms ofFIG. 9 for the case of π/2 phase angle corresponding to the mains cycle.Capacitors Cm and Co filter these high switching frequency signals toprevent them at the input and output, respectively. The maximum voltagestress across the switches is the sum of both the rectified supplyvoltage and the load voltage.

As depicted in FIG. 8, the configuration is characterized by the use oflow frequency storage capacitors rated at voltages substantially lowerthan the rectified supply voltage peak. Such capacitor can be as low as2 μF (i.e. 200 nF/W) and still guarantee constant output power and ahigh power factor. On the other hand, the load voltage must be higherthan the supply peak voltage (e.g. 400V in case of European mainssupply). The rectified input current it never falls to zero due to thecontinuous current flow from the input voltage supply vm directly to thelow frequency storage capacitor Cs.

A fifth embodiment of a driver device 50 e according to the presentinvention is schematically shown in FIG. 5a . This embodiment issubstantially identical to the embodiment of the driver device 50 c, buthere in this embodiment the buck output filter unit 72 is coupled to thefirst switching element 60, i.e. the input terminals 56 a, 56 b of thebuck output filter unit 72 are coupled to the high voltage node 57 andthe intermediate node 59 of the half bridge unit 70. The second inverterLc is provided in this embodiment in the connection between the secondinput terminal 56 b and the second output terminal 56 d of the buckoutput filter unit 72. It should also be noted that bridge rectifier 62′is bidirectional, as opposed to the previous embodiments where thebridge rectifier 62 generally is unidirectional.

A sixth embodiment of a driver device 50 f according to the presentinvention is schematically shown in FIG. 5e . This embodiment issubstantially identical to the embodiment of the driver device 50 d, butin this embodiment the buck output filter unit 72 is coupled to thesecond switching element 61, i.e. the input terminals 56 a, 56 b of thebuck output filter unit 72 are coupled to the intermediate node 59 andthe low voltage node 58 of the half bridge unit 70. Also in thisembodiment the bridge rectifier 62′ is bidirectional like in the fifthembodiment.

Thus, according to the fifth and sixth embodiments the load is connectedat the output of the buck-boost converter (i.e. in parallel to Co andbetween the source electrode of the bottom switching element 61 andground). The low frequency storage capacitor Cs is connected at theoutput of the buck converter (i.e. in series to inductor Lc), whichoperates in bidirectional mode, thereby enable full ZVS operation inboth switches.

FIGS. 10 and 11 show signal diagrams of the third embodiment for anembodiment of a driver device 50 e as shown in FIG. 5a during one mainscycle. For the example of the steady-state waveforms shown in FIG. 10and the high-frequency waveforms at phase angle π/2 from mains cycleshown in FIG. 11 the following values apply: vm=220 Veff, 50 Hz, 1 MHzswitching frequency, Lm=Lc=700 μH, Po=10 W, vo=450V, Cs=2 μF, PF=90%,THD=47%, maximum voltage stress across switches=761V. The term “av”refers to the average component over a switching cycle. d indicates theduty cycle.

The ZVS operation is shown in the high frequency switching waveforms ofFIG. 11 for the case of π/2 phase angle corresponding to the mainscycle. Capacitors Cm and Co filter these high switching frequencysignals. The maximum voltage stress across the switches is the sum ofboth the rectified supply voltage and the load voltage.

As depicted in FIG. 10, this configuration is characterized by the useof low frequency storage capacitors rated at voltages higher than therectified supply voltage peak. Such capacitor can be as low as 2 μF(i.e. 200 nF/W) and still guarantee a constant output power and a highpower factor. The load voltage must be higher than the supply peakvoltage (e.g. 450V in case of European mains supply). The rectifiedinput current it falls to zero and go negative, thus implying the use ofbidirectional rectifier bridges (see FIGS. 5a, 5b ) in case of mainsrectification.

Next, the control method and device according to the present inventionshall be explained. FIG. 12 shows another embodiment of a driver device50 g including a first embodiment of the control unit 64′ (the otherparts of the driver device are schematically indicated by a single block50′). The LED current iLED is measured and compared to a (preset orvariable) reference current iLED_ref in a comparison element 64 a. Thecontrol error err_i is processed in a controller block 64 b (indicatedby PI) resulting in the duty cycle d as manipulating variable. Togetherwith preset switching frequency fs gate driving signals, representingthe control signals S60, S61 for both switching elements 60, 61, areformed in a (gate) driver block 64 c.

FIG. 13 shows a timing diagram for the (gate of the) the switchingelement 60.

The duty cycle is basically related to the control error as typicallydone in a buck converter. Regarding the embodiments of the driver device50 g, a positive control error err_i (to little current) causes anincrease of d and vice versa.

Self stabilizing behavior is achieved by the arrangement of the boostinput filter unit with respect to the terminals of the buck outputfilter unit. If e.g. more power is drawn in average from the input thanis taken from the output, the bus voltage will increase which inresponse will cause the control to decrease d, which in turn will reduceinput power. In same manner the other embodiments can be operated,wherein the meaning of the duty cycle is toggled, i.e. d is to bereplaced by 1−d for the embodiments of the driver device 50 b, 50 d, 50f compared to the embodiment of the driver device 50 a, 50 c, 50 e.Other operation characteristics as the bus voltage and mains current(PF) are generally not explicitly controlled. They result from designand operation choices and tolerances.

In a further embodiment the maximum bus voltage is also explicitlycontrolled by means of the control, in particular by manipulating alsothe switching frequency. While still manipulating d to control theoutput current, fs is increased in response to an increasing bus voltage(as a result e.g. of a high mains voltage or a high output voltage).Alternatively, it is also possible to separately control T_on and T_off,which however will result in a similar switching pattern.

To avoid too high bus voltages (i.e. to avoid over boosting) in case thereference signal iLED_ref is variable and reduced far below its ratedmax. value, in a further embodiment the control enters a burst mode,i.e. switches off the converter periodically at a burst frequency belowfs (e.g. 10 to 1000 times). Furthermore, and alternatively to frequencymodulation for bus voltage control, fs can be used to shape the inputcurrent, either to improve the PF or to better comply with certain kindsof wall plug dimmers.

Still another embodiment of a driver device 50 h including a firstembodiment of the control unit 64″ is depicted in FIG. 14. Compared tothe embodiment shown in FIG. 12 the control unit 64″ additionallycomprises a second controller block 64 d (indicated by PI) resulting inthe switching frequency fs and fs_brst as manipulating variable providedto the (gate) driver block 64 c. The second controller block 64 dreceives as input the reference current iLED_ref, the bus voltage vc,the input current ir, the input voltage vm and the maximum bus voltagevc_max.

According to the present invention a driver device is proposed accordingto which a buck-boost converter is integrated into a synchronous buckconverter. The two ends are connected to a load, e.g. an HV LED load,and to an energy storage unit, e.g. a small low frequency capacitor,which gives both constant LED current and power factors of 0.9 orhigher. The separate capacitor voltage level allows minimizing thestored energy. The buck-boost converter current arranges for losslessinverter switching, which means high efficiency even at miniaturizedinductors.

There are at least three advantageous configurations of the buck-boostintegrated synchronous buck converter that cope with various load andinput voltage ranges including universal mains. All of them can becontrolled over wide load ranges by means of manipulating the duty cycleonly or the switching frequency or by bursting.

The embodiments 50 a, 50 b of the driver device are mainly suited forload (LED string) voltages higher than peak supply voltage, e.g. for120V mains supply and 250V LED strings. The embodiments 50 c, 50 d aremainly suited for load (LED string) voltages far lower than peak supplyvoltage, e.g. for 120V or 230V mains and LED string voltages 10 . . .150V. These embodiments show low root mean square (rms) currents infilters and inverter. The embodiments 50 e, 50 f are mainly suited forapplications like embodiments 50 c, 50 d. These embodiments show reducedvoltage stress on bus capacitor at somewhat increased rms currents infilters and inverter.

In some embodiments a single ZVS half bridge converter and two inductors(i.e. a buck-boost and a buck converter) are provided. The switch nodeis preferably connected to rectified mains via a buck-boost inductor anddiode that enforces discontinuous conduction mode in the buck-boostinverter. The switch node is further connected to a buck inductor.

The buck converter output can be the low frequency storage capacitor orthe load (e.g. a high voltage string of LEDs), depending on theconfiguration. The input power supply, e.g. a rectifier mains, can beconnected at the input or output of the buck-boost converter dependingon the chosen format. Either only the LED current is controlled constant(e.g. by duty cycle) or both LED current and bus voltage are controlledin two loops with manipulating the frequency, too. Preferably, two highfrequency decoupling capacitors for the converter's input and/or outputare provided.

The present invention is applied in consumer and “prosumer”(professional consumer) drivers, as LED drivers, above 2W, e.g. eitherintegrated into a luminaire or external for HV LEDs. Furtherapplications are non-mains isolated professional drivers with relaxedTHD requirement (e.g. 20%) and HV LED string loads.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limitingthe scope.

The invention claimed is:
 1. A driver device for driving an LED unitcomprising one or more LEDs, said driver device comprising: power inputterminals for receiving a rectified supply voltage from an externalpower supply, power output terminals for providing a drive voltageand/or current for driving a load, a half bridge unit comprising a firstand second switching element coupled in series between a high voltagenode and a low voltage node and having a switch node between said firstand second switching elements, a buck-boost input filter unit comprisinga first inductor and a series diode coupled between a power inputterminal and said half bridge unit, a buck output filter unit comprisinga second inductor coupled between said half bridge unit and a poweroutput terminal, an input decoupling capacitor coupled between the powerinput terminals, an output decoupling capacitor coupled between thepower output terminals, wherein the output decoupling capacitor is inparallel with the load, and a control unit for controlling saidswitching elements, and an energy storage unit coupled between either alow power input terminal and said low voltage node of said half bridgeunit, or a high power input terminal and said high voltage node of saidhalf bridge unit; wherein the driver device is configured for zerovoltage switching of the first and second switching elements.
 2. Thedriver device as claimed in claim 1, wherein a high power input terminalis coupled to the high voltage node of the half bridge unit and a lowpower input terminal is coupled to the buck-boost input filter unit. 3.The driver device as claimed in claim 2, wherein an input terminal ofsaid buck-boost input filter unit is coupled to the low power inputterminal and wherein the output terminal of said buck-boost input filterunit is coupled to the switch node of the half bridge unit.
 4. Thedriver device as claimed in claim 1, wherein the input terminal of saidbuck-boost input filter unit is coupled to the energy storage unit orthe low power output terminal and wherein the input and output terminalsof said buck-boost input filter unit are coupled to the switch node ofthe half bridge unit and the low power input terminal.
 5. The driverdevice as claimed in claim 1, wherein said energy storage unit comprisesa charge capacitor.
 6. The driver device as claimed in claim 1, furthercomprising a rectifying unit for rectifying an AC supply voltage, inparticular a mains voltage, into said rectified supply voltage.
 7. Thedriver device as claimed in claim 1, wherein the control unit isconfigured to keep an output current of the half bridge unit constant,the voltage across said energy storage unit below a predeterminedthreshold.
 8. A light apparatus comprising: a light assembly comprisingone or more LED units comprising one or more LEDs; and a driver devicefor driving said light assembly as claimed in claim 1.